1. Field of the Invention
The invention pertains to systems for reading back information recorded on a magnetic medium, such as a magnetic disc or magnetic tape of the type commonly used in computer systems.
2. Description of the Prior Art
The objective of all systems and methods for recording and reading back information on magnetic medium is to record the information with the highest possible density and to read back such information with minimum error. Such information is normally recorded as regions of saturated magnetization in the north pole or south pole sense. Information is contained in the transitions from a region of one sense of saturation to a region of the opposite sense, and in the relative distances between the transitions.
The encoding schemes which are generally recognized to be the most efficient employ one transition per bit of information. A typical example of such encoding methods is one commonly called "non-return to zero" ("NRZ"). Under this scheme, a zero binary bit of information would be a region of one type of saturation and a one binary bit would be a region of the opposite sense of saturation. Thus, a change of saturation between zero and one would show up as a change of magnetization. When the magnetic medium is passed in conjunction with the readback mechanism, voltage pulses would be obtained which correspond to changes in saturation between the north and south directions. Therefore, there is no output from the readback head for a string of like bits. For this reason, to decode the digital data properly, a companion wave form comprising a series of clock pulses placed in conjunction with each bit cell is employed. In high density recording, the only reliable way to obtain a bit clock is to provide information on the data track pertaining to the bit cell boundaries. A commonly utilized method is the use of a synchronizing transition recorded at periodic, determined intervals in the data stream. The synchronizing bit is selected to be the opposite of the last data bit of the preceding bit stream. In a common application, the synchronizing information is recorded after 16 data bits so that at least one transition will occur for each group of 17 bit cells.
In existing systems, read clocks are generated from recorded transitions by synchronizing an oscillator to the transitions in a phase locked loop. To permit generation of a stable output clock and at the same time provide the capability of locking onto the control signal derived from the recorded transitions, existing systems utilize a recorded preamble to the data stream. The preamble ordinarily comprises a lengthy series of a predetermined code which generates a pattern of transitions onto which the phase locked loop can lock, provided that the preamble is sufficiently lengthy. Long preambles are undesirable because they are wasteful and diminish valuable recording space on the magnetic medium.
Therefore, there has been a felt but unfulfilled need for systems which provide full clock synchronization with only one synchronizing pulse.